Biodegradable resins that are decomposed in a natural environment and molded bodies using the resins have been hitherto paid attention to.
Since the biodegradable resins can be produced from natural resources such as corns, there is no anxiety that the resources are exhausted, as compared with synthetic resins using fossil fuel such as petroleum or coal as a material. Further, since the biodegradable resins are decomposed in a natural world, a problem of a deficiency of waste treatment places can be solved. Especially, since the biodegradable resins have properties that the biodegradable resins use the natural resources as materials and are decomposed in the natural world, the biodegradable resins can advantageously suppress a quantity of generation of CO2 gas that is considered to cause the global warming.
Aliphatic polyesters of the biodegradable resins, particularly, polylactic acid ordinarily has a high melting point as high as 170 to 180° C. The molded body made of the polylactic acid usually has a property of transparency and has already begun to be put to practical use depending on a use. The biodegradable resins are employed for materials for agriculture, forestry and fisheries such as films for covering soil, plant pots, fishing lines, fishing nets, materials for civil engineering works such as water holding sheets, plant nets, and a package and vessel field, particularly a filed of disposable products such as packages and vessels that are hardly recycled due to the adherence of soil or food or the like, daily miscellaneous goods, sanitary goods, play goods, etc. A more increase of the uses of the biodegradable resins has been studied from the viewpoint of an environmental protection. For instance, a study has been made that the biodegradable resins are applied to, for instance, television image receivers or acoustic devices, further, casing members of electronic devices such as personal computers or structures such as chassis.
The casing members of the electronic devices or the structures require a heat resistance to about 80° C. or so by taking a heat generation upon starting into consideration.
The molded body made of the polylactic acid is ordinarily poor in its heat resistance and has a glass transition temperature (Tg) of about 60° C. Accordingly, for instance, when temperature exceeds the glass transition temperature as shown in FIG. 1, the modulus of viscoelasticity of the molded body is undesirably deteriorated and deformed. Therefore, in order to employ the molded body for uses requiring the heat resistance, various examinations have been carried out. The heat resistance referred herein means a heat resistance having an adequately high modulus of viscoelasticity as high as 200 MPa at about 80° C.
To enhance the heat resistance of the biodegradable polyester including the polylactic acid, for instance, an addition of an inorganic filler has been examined. As the inorganic filler, talc or mica having the heat resistance has been examined. The addition of the inorganic filler is similar to a treatment that, so to say, reinforcing steels are inserted into concrete, and aims to improve mechanical characteristics and harden the resin by adding the hard inorganic filler having the heat resistance to the resin. However, only the addition of the inorganic filler is insufficient.
The polylactic acid exemplified as a typical example of the biodegradable resins is a polymer capable of having a crystal structure. Ordinarily, the polylactic acid is heated and molten at temperature exceeding the melting point of the polylactic acid and a metal mold of temperature not higher than the glass transition temperature (Tg) is filled with the obtained polylactic acid to harden the polylactic acid. Thus, the molded body having a desired form is obtained. In the molded body obtained in such a way, most of the polylactic acid remains amorphous and is liable to be thermally deformed. Thus, a study has been made that after the polylactic acid is molded, the biodegradable resin is thermally treated at temperature exceeding Tg to crystallize the polylactic acid and improve the heat endurance of the molded body. As a result of such a thermal treatment, the mechanical characteristics of the molded body made of the polylactic acid are more improved as shown by B in FIG. 1 than those of a molded body in which the thermal treatment is not carried out as shown by A in FIG. 1. However, since it takes long time for the thermal treatment, an efficient molding operation cannot be carried out. Further, when a thermal treatment temperature is raised, the crystallization of the polylactic acid can be saturated and completed in a short time. However, during the heat treatment, the molded body is undesirably deformed. Accordingly, to prevent the deformation of the molded body, the thermal treatment is carried out at temperature slightly exceeding Tg of the polylactic acid, for instance, at the temperature of about Tg+10° C., so that it takes much time for the thermal treatment to crystallize the polylactic acid. Thus, a method has been required that a large quantity of the molded body made of the biodegradable resin excellent in its heat resistance can be rapidly produced in industrial production processes.
Further, when the polylactic acid is crystallized by an ordinary method, the size of a crystal is in the order of micron to sub mm or so. The crystals themselves of the polylactic acid result in factors of a light scattering to become opaque and a transparency is lost. To solve the problems, that is, to accelerate the crystallization, an addition of, what is called, a nucleus agent is examined.
In order to accelerate the above-described crystallization of the biodegradable polyester, the inventors of the present invention study a resin component in which the crystallization of a polyester that may have a crystal structure such as the polylactic acid is accelerated. As the resin component of this type, the inventors of the present invention propose resin components as described respectively in the specifications of Japanese Patent Application Nos. 2002-038549, 2002-134253, 2002-263279, 2002-263283, and 2003-027590.
Here, the nucleus agent used for accelerating the crystallization of the biodegradable polyester serves as a primary crystal nucleus of a crystalline polymer to accelerate the growth of the crystal of the crystalline polymer. In a broad sense, the nucleus agent may serve to accelerate the crystallization of the crystalline polymer. That is, a material for accelerating the crystallization speed itself of the polymer may be also referred to as the nucleus agent. When the nucleus agent like the former is added to the resin, the crystals of the polymer become fine so that the rigidity or the resin is improved or the transparency is improved. Otherwise, when the biodegradable polyester is crystallized during molding the biodegradable polyester, since all the crystallization speed (time) is accelerated, a molding cycle can be advantageously shortened.
The above-described effects can be found in other crystalline resins as actual examples. For instance, when the nucleus agent is added to polypropylene (refer it also to as PP, hereinafter.), the rigidity or the transparency of polypropylene is improved. Today, the PP whose materiality is improved is put into practical use for many molded bodies. The nucleus agent includes, for instance, a sorbitol type material. Its mechanism of action is not completely clarified, however, a three-dimensional network formed by this material is considered to effectively act. Further, metallic salt type nucleus agents are also put into practical use for the PP. As such metallic salt type nucleus agents, for instance, hydroxy-di(t-butyl benzoate) aluminum, sodium bis (4-t-butylphenyl) phosphate, sodium methylene bis (2,4-di-t-butylphenyl) phosphate, etc. may be exemplified.
As the nucleus agent of the aliphatic polyester, a sorbitol type material as described in Japanese Patent Application Laid-Open No. hei 10-158369 has been examined. There is a description that the above-described material has actual results as a crystallizing nucleus agent for the PP and also effectively acts when this material is added to the polylactic acid. Further, as methods for accelerating the crystallization by adding the nucleus agent to the polyester, various methods have been studied. For instance, a technique as disclosed in Japanese Patent Application Laid-Open No. hei 9-278991 that at least one kind of a group of compounds including aliphatic carboxylic amide, aliphatic carboxylic salts, aliphatic alcohol and aliphatic carboxylic esters and having a melting point of 40 to 300° C. is added to an aliphatic polyester as a transparent nucleus agent, a technique as disclosed in Japanese Patent Application Laid-Open No. hei 11-5849 that at least one kind of organic compound selected from a group of organic compounds having a melting point or a softening point of 80 to 300° C. and a melting entropy of 10 to 100 cal/K/mol is added to an aliphatic polyester as a transparent nucleus agent, and a technique as disclosed in Japanese Patent Application Laid-Open No. hei 11-116783 that an aliphatic ester having a specific structure as a clarifying agent is added to a polylactic acid resin have been proposed.